In A Nutshell
The kraken and Leviathan are often considered vaguely interchangeable sea monsters, but they’re actually very different creatures with many misconceptions built up around them both, largely because of the influence of pop culture. (Sorry, Liam Neeson, but the kraken wasn’t even actually Greek.) The kraken was a squid-like monster from Norse mythology said to be so large it would attack ships and most likely born from real sightings of giant squid. Leviathan was a whale-like monster from Hebrew folklore.
The Whole Bushel
Leviathan and the kraken are two sea monsters from folklore, and as such, they’re often used somewhat interchangeably. Wrongly so, as they’re completely different creatures with completely different origins. And one is even definitely rooted in truth.
Leviathan was originally a part of Hebrew folklore. He was first described in the book of Job as a whale-like creature with an armored back that was impervious to weapons. He was also the legendary creature that swallowed Jonah. It was God that forced him to vomit Jonah back up onto the land, but according to Rabbinical literature, it was God that created him, too—and there were originally two. Leviathan was created on the fifth day, and originally, there was a male and a female. God realized that the potential for offspring as massive and as dangerous as they were could mean the end of the world, so the female was killed. (Her meat was said to have been preserved for a feast that would be held to celebrate the advent of the Messiah.)
The massive whale could breathe fire, and it was said in the original texts about him that in order to satisfy his hunger he would breathe fire to boil the seas and kill all the marine life he needed to satisfy his appetite.
In later texts, it was said that when Gabriel returned to Earth, it was this surviving Leviathan that would be killed in order to provide food for the feast. Leviathan was also the monster in the original version of the legend that sailors would be lured into landing on what they thought was an island in the middle of the ocean, but was actually the giant whale’s back; once they were on him, he would disappear beneath the waves and take them with him.
The kraken was born in Scandinavian mythology, and is one of the few mythological monsters that have been proven to be based on at least partial fact. Original stories of the kraken tell of a tentacled monstrosity more along the lines of an octopus than of a whale; in fact, it’s been surmised that the original stories were based on sighting of giant squid.
There’s a story featuring the kraken that’s very similar to the story of Leviathan tricking sailors into landing on his back. In the kraken version, however, the sailors believe that they have come across a series of small islands, which then turn out to be the kraken’s body and tentacles that surface completely when the ships get within reach, grabbing them and pulling them beneath the waves.
The kraken made the jump from mythology to zoology as early as 1752, when a Scandinavian bishop penned the Natural History of Norway and included the kraken. Later, giant squid washing up on beaches provided more concrete evidence that there indeed were giant, kraken-like monsters in the ocean.
Signs of even larger, earlier giant squid have been found, but concrete evidence is tricky. Unlike the whales of the Leviathan legends, the kraken would have been, by description, a soft-bodied cephalopod that wouldn’t have survived centuries in any form, not leaving behind fossil evidence that can be so telling.
Show Me The Proof
The Encyclopedia of Demons and Demonology, by Rosemary Guiley
LiveScience: Lair of Ancient ‘Kraken’ Sea Monster Possibly Discovered
LiveScience: What Is a Kraken?
Jewish Encyclopedia: Leviathan and Behemoth